Polyamide resin compositions reinforced with fibrous reinforcing materials such as glass fiber and carbon fiber or inorganic fillers such as calcium carbonate are widely known. However, since these reinforcing materials have poor affinity to polyamide, the reinforced polyamide compositions have reduced toughness while mechanical strength and heat resistance are improved. Further, molded articles of a fiber-reinforced polyamide resin composition suffer from appreciable warp. Besides, in using the inorganic fillers, substantial improvement in mechanical strength or heat resistance cannot be obtained unless they are added in a large quantity.
In order to eliminate these disadvantages of conventional reinforced polyamide, resin compositions comprising polyamide and a layered silicate typified by montmorillonite have been proposed as disclosed in JP-A-62-74957, JP-A-63-230766, JP-A-2-102261, and JP-A-3-7729 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
These proposed resin compositions are composite materials in which polyamide chains enters the spacings between the layers of a layered silicate so that the layered silicate is uniformly dispersed on the molecular level. In using montmorillonite as a layered silicate, it is indispensable, as described in the above publications, to treat montmorillonite with a swelling agent, such as an ammonium or onium salt of an aminocarboxylic acid, to widen the spacing between layers before compounding with polyamide or the monomer providing polyamide.
In this situation, the inventors of the present invention previously proposed that a polyamide composite excellent in mechanical strength and heat resistance can be obtained without pretreatment with a swelling agent such as an aminocarboxylic acid by polymerizing a polyamide-forming monomer(s) to which specific swelling fluoromica has been added (JP-A-6-248176). However, the polyamide composition containing the swelling fluoromica as well as the polyamide composition containing montmorillonite, one of layered silicates, still have the problem that the molded articles obtained therefrom have a lower weld strength than those obtained from polyamide only.